1. Field of the Invention
This invention relates generally to magnetic recording disk drives, and more particularly to ultrahigh data density disk drives.
2. Description of the Related Art
Conventional magnetic recording disk drives use “continuous media” disks with a recording layer of a continuous magnetic film. Data is recorded or written in the continuous film in concentric data tracks when regions of the film are magnetized in the circumferential or along-the-track direction by an inductive write head, resulting in written data “bits”. Disk drives have been proposed that use “patterned media” disks with a recording layer that is patterned into concentric data tracks, with each track having discrete magnetic islands or “bits” separated by nonmagnetic regions. Data is written in the discrete islands in the previously patterned data tracks by an inductive write head that magnetizes the magnetic material in the islands, resulting in the written data “bits”.
In either type of disk drive, the written data bits are read back when the magnetic fields from the bits are detected by a magnetoresistive read head and processed by the disk drive's circuitry into data as the disk rotates past the read head. The read head is a stack of thin films formed between two shields on a planar surface of a head carrier, with the planar surface being oriented perpendicular to the disk and parallel to the cross-track or radial direction. The shields sharpen the spatial response of the read head in the down-track direction, improving the down-track resolution. The spatial response in the cross-track direction is largely determined by the lithographically determined width of certain layers of the read head's thin film stack. The conventional read head has an inherent asymmetric spatial resolution, meaning a high spatial resolution in the transverse direction perpendicular to the planar surface (the along-the-track or circumferential direction) and a lower spatial resolution in the direction parallel to the planar surface (the cross-track or radial direction).
The need for disk drives with ever increasing data density requires that the data bits be packed closer together on the disk. The ratio of linear bit density in bits per inch (BPI) in the along-the-track direction to the track density in tracks per inch (TPI) in the cross-track direction is called the bit aspect ratio (BAR). Because of the inherent asymmetry of the conventional read head, and the challenges of implementing suitable track-following servo systems to handle very narrow tracks, the BAR of conventional disk drives has typically been larger than 5.
To achieve ultrahigh data density, e.g., on the order of 300 Gbits/in2 and above, further increases in BPI are made more difficult because the requisite scaling down of media grain size can result in a thermally unstable recording medium. Therefore, further increases in density with conventional media are likely to be accomplished primarily by increasing TPI, which results in decreasing BAR. Disk drives using patterned media are likely to have BAR values near 1, due to lithographic patterning challenges that make media fabrication considerably more difficult at BAR values larger than approximately 2.
What is needed is an ultrahigh data density magnetic recording disk drive with a read head that can detect bits packed on the disk with a low BAR.